Potentially overkill A2W Heatpump Suggested

[Gaf]

New build, 218m2. UFH and MVHR. Max preferred indoor temperature of 21C (likely to be 20C but going high for the purpose of selecting the HP).

 

Professionally calculated Heat Loss Indicator (HLI) of 1.01 and ~30 kWh/m2/yr. Irish BER rating of A2.

 

A rough heat pump calculator from an Irish crew indicates a required kW HP of 5.3kW based on internal temp of 21C, external temp of -3C, and HLI of 1.01.

 

Two HPs have been recommended, one is 11.4kW (Thermia iTec Eco) and the other is 12kW (Mitsubishi Ecodan). Plan to talk to the builder about the recommendations, so far he's on the ball with his approach to things; but I like being informed about these things just so I go in with eyes open and understand the needs/specs required.

 

Curious if the recommended kW of both machines seems overkill?

 

 

[JohnMo]

I would do your own heat loss calculations and see where you fall. Sounds big.

 

But HLI of 1.01 and a min design temp of -3 would give a heat load of a little over 5kW (if I am reading the parameter correctly), which would put you at 6kW heat pump output at say -3 allowing for DHW.

[joth]

Overkill for heating only, but what is your plan for DHW? Often the ASHP gets oversized just to reduce cylinder reheat times (esp large family, lots of rooms/bathrooms, small tank)

 

If you do oversize it's more vital than ever you don't zone the UFH but run it all at one target temperature, to maximise emitter size / throughput while keeping flow temperature low as you efficiently can.

Will all 218m2 floor area have UFH or just some part of it?

[PhilT]

What are the minimum temps likely in your area? In my area in Dec this winter it was -7, and in Dec '22 it was -12 on two consecutive nights, so very glad indeed that my heat pump is at least 50% oversized, but despite this performance is still way better than design spec. 

[Conor]

Your total heat load is a little lower than ours. We've a 9kW and it seems to cope well. Need to check the minimum load on the pumps as that's the main factor, not the maximum output. I.e. how low can it modulate down to.

[ReedRichards]

The maximum output of your heat pump would seem to be way more than you need but the parameter you need to worry about is the minimum output.  The higher the minimum output the more time your heat pump will spend cycling between on and off.  That's likely to be less efficient than if your heat pump is on continuously heating you water to a lower temperature.

[Gaf]

@joth  All 218m2 will have UFH. The plan was to have each room zoned, will it be OK to keep the zoned system but keep each room temperature the same? Would that be essentially the same as having it all running at one target temperature?

 

House will have a private well with a pumped system under pressure. The house will initially have two adults and two kids under 3 years (potentially a third later), and this will be the house they all grow up in, so want to future proof. There are three bathrooms, all with showers in the house; and I hate electric showers so would like a system that could manage hot water for showers without electric showers.

 

Plan from the architect was for a hot water tank placed on ground floor under the stairs to include a pump for DHW around the house, I assumed the ASHP would heat the water in this also but I hadn't considered this need in my original calculations.

 

@PhilT Lowest temperature for '22 and '23 was -4C.

 

@Conor @ReedRichards  Just to check, is "minimum load" and "minimum output" the same thing? What's the term that manufacturers use for this? Attached is the Thermia iTec Eco datasheet for the 12kw and I can't make out what part of the spec refers to minimum load.

 

Thermia_ITec_Eco_datasheet_ENG_25_02_2021-1.pdf

[sharpener]

16 hours ago, Gaf said:

All 218m2 will have UFH. The plan was to have each room zoned, will it be OK to keep the zoned system but keep each room temperature the same? Would that be essentially the same as having it all running at one target temperature?

 

Your HP sizing sounds massive for a 218m^2 ?well insulated new build house.

 

What is purpose of zoning? Probably best to keep whole house warm all the time on weather compensation, with perhaps a 2 or 3 deg setback at night, then you don't need the complications of multiple thermostats and actuators.

 

However many ppl don't like bedrooms warmer than 18C, so maybe you should have those on a separate manifold.

[ReedRichards]

I would have thought that "minimum load" would mean the lowest power that the heat pump could draw.  I'm not sure about "minimum output".  You're likely to be running at minimum load when it's relatively warm out so you could b achieving quite a high COP, so a decent output as a multiple of the load.  Conversely, if it's very cold out the load will be high but the output might not be that much higher because the COP would be low.  So I'm not sure when you would encounter a situation of minimum output.  

[JamesPa]

18 hours ago, sharpener said:

 

Your HP sizing sounds massive for a 218m^2 ?well insulated new build house.

 

What is purpose of zoning? Probably best to keep whole house warm all the time on weather compensation, with perhaps a 2 or 3 deg setback at night, then you don't need the complications of multiple thermostats and actuators.

 

However many ppl don't like bedrooms warmer than 18C, so maybe you should have those on a separate manifold.

+1.

 

Mine is only very slightly smaller at 205 sq m.  Its a part extended and only partially insulated 1930s solid wall job with suspended floors.  7.5kW is the measured demand at -2.  Most of the season it sits close to 4kW demand, so any heat pump needs to modulate down to this figure to be suitable.

 

As others have suggested its possibly being sized for DHW reheat, but do you really need the speed of reheat that 10-12kW would give (thats the equivalent of 3.5 immersion heaters)?  Even if you do wouldn't it be better just to add 50l to the cylinder size, or, if its rarely that this is needed, switch on the immersion as well?    The British obsession with infinite amounts of instant hot water never ceases to amaze me!

[Conor]

On 21/02/2024 at 21:05, Gaf said:

 

@Conor @ReedRichards  Just to check, is "minimum load" and "minimum output" the same thing? What's the term that manufacturers use for this? Attached is the Thermia iTec Eco datasheet for the 12kw and I can't make out what part of the spec refers to minimum load.

 

Thermia_ITec_Eco_datasheet_ENG_25_02_2021-1.pdf 1.18 MB · 2 downloads

Min load is the lowest the heatpump can modulate down to. I.e. the lowest power it can consume continuously. This is different to the start-up load that is often quoted. Min output is probably highly variable and dependent on different factors. Heatpumps are rated in kW of their output at defined sCOP, not their total electrical consumption.

[SteamyTea]

15 minutes ago, ReedRichards said:

I would have thought that "minimum load" would mean the lowest power that the heat pump could draw.  I'm not sure about "minimum output".

It is down to clarification of terminology.

I have not found a simple way to explain the difference.

The way I see it is 'load' is the power, or energy, needed to heat something to the desired temperature.

While 'output' is how much power, or energy, can be supplied at any given time.

There may seemingly be a mismatch between the two, and that is when detail comes into it i.e. run the system for longer or shorter times.

[sharpener]

"Load" is traditionally the term used in electrical engineering to describe anything that absorbs power. So the house is the load for your heat pump.

 

"Output" is the term for quantifying a source of power.

 

They are both measured in kW. In an HP installation the control system ensures the HP output is matched to the load by modulating the compressor and fan speed to maintain (typically) a 5C temperature difference between the flow and return pipes.

 

If the load is greater than the max output, the HP will run at max but achieve less than 5C delta T. This is OK for the HP but the heating circuit will not reach the design temperature so the emitters will not reach their rated output and the house will be too cold.

 

If the load is less than the minimum that the HP can modulate down to then it will cycle on and off to maintain an average output figure that matches the load. Provided there is sufficient system volume for this to happen not more than 3 cycles per hour it is OK, more cycling than that is inefficient and also bad for compressor lifetime. Zoning the house into a large number of individually controlled areas is likely to result in more frequent cycling so is discouraged.

 

The minimum output figures are on the data sheet immediately below the pictures. So the 12kW model will modulate down to 3.4kW. But the 8kW will go down to 1.7 which is a significantly wider range and the 5kW down to 1.1 which is about the same ratio.

 

On 17/02/2024 at 16:04, Gaf said:

New build, 218m2. UFH and MVHR.

 

With this spec you can see from this cheat sheet that you should be thinking in terms of a HP in the 4 - 8kW range. So the 8kW model is almost certainly going to be big enough for you and it is likely you could get away with the 5kW.

 

Overspecifying it in terms of high room temps, low OAT (Ireland is mostly quite temperate) or caution on the part of the builder will result in higher capital cost, higher running cost and possibly a shorter lifetime. Better to err on the small side and aim to use supplementary heating on the few really cold days a year, either the built-in boost heater, a fan heater or even leave the oven on with the door open (perhaps not with children in the house!).

 

 

Edited February 23 by sharpener

[JamesPa]

On 21/02/2024 at 21:05, Gaf said:

Just to check, is "minimum load" and "minimum output" the same thing? What's the term that manufacturers use for this? Attached is the Thermia iTec Eco datasheet for the 12kw and I can't make out what part of the spec refers to minimum load.

Heat pump manufacturers usually produce tables of min. max load as a function of flow temperature and outdoor temperature.  Both can vary a lot with those parameters, and proper design involves use of the tables as some heat pumps only achieve their 'Max' (or Min) when operated at very specific conditions, which may not correspond to the conditions you have. You should ideally obtain the tables.  Here is an example from Mitsubishi.

 

 

 

[Gaf]

Coming back to this as thing are moving along with the build and it's being suggest we include a combi-boiler with the A2W. From the points made by everyone, it seems we should:

1. Reduce the size of the proposed A2W
2. Zone the whole ground floor as one and the whole first floor as one, and don't zone each room separatel

From Googling, it doesn't seem like it's a requirement to have a boiler when you also have an A2W heat pump. There will be 4 people living in the house (2 adults, 2 kids). It's been suggested to include a 200L combi-boiler.

 

Curious what the thoughts are on having both an A2W heat pump and a combi-boiler?

 

 

[PhilT]

37 minutes ago, Gaf said:

Coming back to this as thing are moving along with the build and it's being suggest we include a combi-boiler with the A2W. From the points made by everyone, it seems we should:

1. Reduce the size of the proposed A2W
2. Zone the whole ground floor as one and the whole first floor as one, and don't zone each room separatel

From Googling, it doesn't seem like it's a requirement to have a boiler when you also have an A2W heat pump. There will be 4 people living in the house (2 adults, 2 kids). It's been suggested to include a 200L combi-boiler.

 

Curious what the thoughts are on having both an A2W heat pump and a combi-boiler?

 

 

does your property qualify as a "self build"? i.e. has it "been built mainly using the labour or resources of the first owner", and "not been owned by a business or organisation"?

If so you would qualify for £7,500 BUS grant, but you would not be able to have any kind of fossil fuel energy source.

 

One other question that I can't see commented on so far is that you want a degree of future proofing (per your 2nd post above).

[JamesPa]

1 hour ago, Gaf said:

Coming back to this as thing are moving along with the build and it's being suggest we include a combi-boiler with the A2W.  ...  It's been suggested to include a 200L combi-boiler.

er why?   There is no need at all.  And anyway what is a '200l combi boiler'  Sofaik 'combis' are gas boilers, which are sized in kW not l?  And the whole point about combis is that they dont store water so 200l makes absolutely no sense.

 

Both will fail if electricity fails, so the only 'redundancy' you are getting is against failure of the heat pump itself.  If your tank has an immersion heater that deals with DHW, if you want a degree of redundancy in the space heating then include a 3kW willis heater in the system for about £100, or fit the heat pump manufacturers backup heater if they do one.  Or buy a few fan heaters for the rare occasion this is going to happen.  All of these will also fail if the electricity fails but unless you have an open fire, a wood burner, or something that doesn't rely on a pump or fan (almost nothing these days), this is unavoidable.

 

Whoever suggested this is obviously scared of heat pumps (and apparently doesnt know much about boilers either).  Hope they aren't also specifying and/or fitting it.

Edited May 25 by JamesPa

[JamesPa]

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Edited May 25 by JamesPa

[JohnMo]

2 hours ago, Gaf said:

Coming back to this as thing are moving along with the build and it's being suggest we include a combi-boiler with the A2W. From the points made by everyone, it seems we should:

1. Reduce the size of the proposed A2W
2. Zone the whole ground floor as one and the whole first floor as one, and don't zone each room separatel

Sounds like you are being advised by people you shouldn't really listen too. 

 

An ASHP on its own would do hot water and all the heating you need for the house. I would based on kids being in the house do a 300L heat pump cylinder. Then install a heat pump that can 6kW at your coldest temperature. Depending on manufacturer that will be a 6 to 8kW unit. That will give you 3 hrs to DHW on the coldest day. Anything else is too big.

 

Design the system to run one flow temperature. Have electric heat mats and electric towel rail in upstairs bathrooms.

 

Your system becomes very simple -

ASHP, 3 port diverter valve. One side is connected to UFH manifold (no mixer or pump or actuators), and radiators (with rtv). The other side is connected to cylinder coil. Use the heat pump controller to control temperature.

 

Gas boiler, if you already had one installed you could use it, but to pay an additional £1000 and have the additional complications makes no sense.

[Gaf]

@JamesPa @JohnMo Cheers lads. I'll reserve judgement on the contractor until I chat to him again as it's possible he was describing how the cylinder is 'like' a combi-boiler in that it's a 'unit' rather than a traditional hot press-type water cylinder. The original plan was to have a standalone DHW tank but this has changed due to it not fitting in the position the architects put it on the drawings.

 

Can I check if I've understood things properly? The suggested model is the Thermia iTec Eco. The brochure for this model does say that "Each iTec Eco is delivered with an indoor unit. Depending on your requirements, you can select one of five different indoor units and customize your installation."

 

So the setup would be:

1. A2W Heat Pump (located outside) - Likely Thermia iTec Eco
2. Heat Pump Cylinder (located inside) - Likely Thermia iTec Eco "Total" (180L) or "EQ" (240L)

 

 

 

 

 

 

 

 

[JohnMo]

Not really a fan of manufactures that are really offering a monobloc ASHP, but purposely split it, to ensure they get you to purchase everything from them.

 

Heat pump life say 15 years, cylinder should last a life time, what happens when you heat pump needs to be replaced, do you also have to replace the cylinder?

[JamesPa]

1 hour ago, JohnMo said:

Not really a fan of manufactures that are really offering a monobloc ASHP, but purposely split it, to ensure they get you to purchase everything from them.

 

Heat pump life say 15 years, cylinder should last a life time, what happens when you heat pump needs to be replaced, do you also have to replace the cylinder?

I have the same prejudice.  Cylinders are readily available and understood by any plumber.  Monobloc heat pumps and the bits that go immediately with them are a bit specialist.  So keep them separate in my personal opinion.    

 

The wall mounted unit (as opposed to the one with the integral cylinder) does keep them separate, albeit splitting the monobloc (some call this arrangement a 'hydrosplit')  Really its just a box for the pump, control unit and diverter valve which is OK by me.  The control unit and diverter valve are pretty much always separate (and sensibly so) and its not unknown for the pump to be separate (Mitsubishi, for example, don't the pump in the external unit).  So putting these three components together in a metal box isnt silly

 

One further thing to note about the 'total' - like a 'pre plumbed cylinder' its possibly a 'cover' for the intent to use rookie plumbers.  By using a pre-plumbed cylinder a few of the possible mistakes that rookie plumbers can make are avoided.

 

This is all personal preference stuff however and considerations of the available space.  If you want an 'airing cupboard' as a by product of DHW, you are going to want a separate cylinder.